Switch from fetal to adult hemoglobin is associated with a change in progenitor cell population.

To examine the switch from fetal to adult hemoglobin at the cellular level, erythroid progenitor cells from newborn infants and adults were cultured in methyl cellulose with erythropoietin. Individual erythroid colonies were labeled with [3H]leucine at various times, and globin synthesis patterns examined by gel electrophoresis and fluorography. The percent gamma- or beta-globin synthesis was determined from the total of gamma + beta, and the percent G gamma from the total of G gamma + A gamma. The nonparametric correlation coefficients of percent G gamma with percent gamma or beta were obtained. Each group of colonies at each time point was examined separately. In colonies from adult blood, the proportion of G gamma-synthesis did not correlate with the proportion of gamma-synthesis. Colonies from newborn blood fell into two groups. Those that developed from relatively mature progenitor cells, and were seen on day 14, showed a strong negative correlation of G gamma with beta-globin synthesis. However, those newborn colonies that developed from immature progenitors, and were seen later in culture (days 17 and 21), showed no correlation of G gamma with beta-synthesis. These findings are compatible with a clonal model for hemoglobin switching. Fetal progenitors, in which G gamma- and beta-syntheses are negatively correlated, are gradually replaced during ontogeny by adult progenitors. The adult progenitors produce more beta (less gamma), and the proportions of G gamma- and gamma- or beta-synthesis are not correlated.

Investigations of the simian ontogenic switch from fetal to adult hemoglobin at the progenitor cell level.

The ontogenic switch from fetal to adult hemoglobin could result from discontinuous events, such as replacement of fetal erythroid progenitor cells by adult ones, or gradual modulation of the hemoglobin program of a single progenitor cell pool. The former would result in progenitors at midswitch with skewed fractional beta-globin synthesis programs, the latter in a Gaussian distribution. For these studies, we obtained bone marrow from rhesus monkey fetuses at 141-153 d (midswitch). Mononuclear cells were cultured in methyl cellulose with erythropoietin, and single BFU-E-derived colonies were removed and incubated with [3H]leucine. Globin synthesis was examined by gel electrophoresis and fluorography. The beta-globin synthesis pattern of single fetal colonies was skewed, and did not fit a normal distribution. The fetal pattern resembled the pattern of an artificial mixture of fetal and adult progenitors, suggesting that the fetal progenitor pool could contain populations with different beta-globin programs. This non-Gaussian distribution in the progenitors of midswitch fetuses is consistent with a discontinuous model for hemoglobin switching during ontogeny.

Effects of hemin on erythropoiesis.

It is clear that in vitro hemin increases the number of blood BFU-E derived colonies from normal donors. This occurs with sickle donors as well, despite the increased levels of hemin in vivo in these patients. The effect of hemin on relative gamma globin synthesis is inconsistent, however. In a few cases, delayed addition of hemin led to increased gamma globin synthesis. In time course studies of cultures from normal donors, hemin added on day 0 shifted the day of peak colony number from 13-14 to 16-20 days. The temporal decline in gamma globin synthesis was not altered. In cultures from sickle donors we found that the time for maximal colony number was later than in normals, occurring at 16-20 days even without hemin, and was not further delayed by hemin. The relative proportion of gamma globin synthesis was higher on day 14 in the sickle than the normal cultures, and the temporal decline was somewhat slowed in the sickle cultures by hemin. The elevated gamma synthesis and the later time for peak colony growth in the sickle cultures suggest that the erythroid progenitors in the blood of the sickle patients are less mature than those from normal individuals. There are several possible explanations for the detection of increased numbers of colonies in cultures containing hemin. Hemin may delay the final maturation of erythroblasts within erythroid colonies, thus shifting the time of maximal growth. It may also increase the extent of final maturation, leading to more complete hemoglobinization of the erythroblasts within the colonies, and thus increasing the number of colonies that are eventually recognized as erythroid.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of hemin in vitro and in vivo on human erythroid progenitor cells.

Hemin stimulates erythropoiesis and hemoglobin synthesis in vitro. We cultured erythroid progenitor cells from normal individuals, patients with sickle cell anemia, and a patient with acute variegate porphyria who received intravenous hemin treatment, with 0-800 microM hemin added in vitro. Fifty to 200 microM hemin consistently stimulated colony growth from normal donors 2- to 8-fold, while concentrations of up to 400 microM were stimulatory in cultures from donors with sickle cell anemia. In vivo hemin decreased the number of blood BFU-e in the patient with porphyria, but did not abrogate the in vitro stimulatory effect of hemin. Hemin concentrations which increased colony numbers increased gamma globin synthesis in some studies and decreased it in others. Hemin thus has clearcut erythroid growth-potentiating activity, although a consistent effect on globin chain regulation is not apparent.

Adult 'fetal-like' erythropoiesis characterizes recovery from bone marrow transplantation.

The transient fetal-like erythropoiesis which appears during recovery from bone marrow transplantation has now been examined at the level of erythroid progenitor cells. A 7-year-old boy with beta +-thalassaemia major was studied during engraftment from his beta-thalassaemia trait sister. Hb F and i antigen rose as expected. Macrocytosis never developed, but red cell size distribution became very heterogeneous. Bone marrow CFU-E and BFU-E were detected by 30 d, prior to the appearance of reticulocytes. Marrow erythroid progenitor cell numbers were normal by 146 d, while those in the blood became normal by 360 d. After transplantation globin synthesis ratios in erythroid colonies were diagnostic of thalassaemia trait, indicating engraftment. Individual erythroid colonies derived from both blood and marrow at all times during reconstitution showed no correlation of G gamma and gamma. Thus the fetal-like stress erythropoiesis of marrow expansion following transplantation was derived from adult and not fetal progenitor cells.

Specific globin mRNAs in human erythroleukemia (K562) cells.

Specific globin mRNA accumulation was quantitated in several lines of K562 cells in the absence and the presence of hemin. Using specific cloned DNA probes, the amounts of zeta, alpha, epsilon and gamma mRNAs were shown to be increased 2-3-fold in the presence of 20 microM hemin. No delta- or beta-globin mRNAs were detectable in any of the lines. In one line, Bos, there was a marked decrease in epsilon-globin mRNA, which increased with hemin, although still to much lower levels than in the other lines. The decreased epsilon-globin mRNA accumulation in Bos is shown to be due to decreased epsilon-globin gene transcription.

Fetal hemoglobin synthesis in erythroid cultures in hereditary persistence of fetal hemoglobin and beta o-thalassemia.

To determine whether hemoglobin regulation is normal in diseases affecting beta-globin gene expression, globin synthesis was examined in members of a family of a patient with hereditary persistence of fetal hemoglobin/beta o-thalassemia (HPFH/beta o-thal). The HPFH defect is the Ghanian type II, with a deletion from psi beta 1 to at least 20 kb 3' to beta. The beta o-thal gene has the haplotype II restriction enzyme pattern and has the beta 39 nonsense mutation. Erythroid colonies from blood BFU-E were radiolabeled, and globin chains were separated by gel electrophoresis. Colonies from the beta o-thal heterozygote had non-alpha/alpha ratios more balanced than in the reticulocytes. Gamma synthesis was 11% of non-alpha, which is higher than in reticulocytes, but within the range seen in normal adult colonies. Both HPFH heterozygotes produced 20%-30% gamma in erythroid colonies as well as reticulocytes, although non-alpha/alpha was more balanced in the colonies. The HPFH/beta o-thal patient produced 100% gamma in reticulocytes and in colonies. G gamma and gamma-synthetic proportions were not correlated at the individual colony level in the heterozygotes, suggesting that they had "adult" and not "fetal" progenitor cells. The Hb expression of these adult progenitors is presumably modulated normally in vivo in beta o-thal, but the normal decrease in HbF production does not occur in gene deletion HPFH.